Twenty-one shillings (6)
I believe the answer is:
guinea
'twenty-one shillings' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for guinea that I've seen before include "One pound and fivepence" , "Old money and pig" , "Sum of PS1.05" , "Former British coin worth 21 shillings" , "105p in old money" .)